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Commodore 64 Breadbin Help - Bad Ram?


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Hello everyone, just got a c64 in terrible shape. Broken case during shipment among other things. It does boot to a normal screen expect that the ram available only shows about 31k, instead of the close to 39k that should be available. Disk programs don't load correctly and cartridge games don't play either for the most part. Radar rat race seems to but Dig Dug just hangs. My question is if the problems that are occurring are due to a single bad ram chip perhaps? Or could there be more than just that one problem? I don't own a dead test cartridge and don't have any real way of knowing what chip is bad. Can someone recommend me a way to trouble shoot with having to buy a dead test or other cart. I have an easy flash 3 but it doesn't seem to want to boot. Also if it is indeed ram, can someone recommend me a place to buy a single or small lot of chips. Thanks everyone for their time!  

Edited by INTVCruise
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I wonder if FastLoad bypasses the regular RAM test.  IIRC, the RAM test in the Kernal sets the top of BASIC at the first address that fails to return the same value as what is stored.  FastLoad maybe skip that and make some assumptions.

 

That said, if some cartridges work and others do not, it could still be RAM.  Radar Rat Race may not rely upon RAM in the upper regions which appear to be bad, while Dig Dug might.

 

I believe it is possible, though I am not confident, the problem could be the PLA.  My troubleshooting skills on the 64 are about 20 years rusty -- fortunately, I have not needed to troubleshoot any of mine beyond the standard of a completely dead CIA or PLA.

 

Some disk software may load just fine if the programs do not load up past the 31k mark.  So, something like a simple short BASIC program might work.  Otherwise, something which loads up to, say, 49152, may fail as that is well over the 31k mark.  If BASIC works, you could try POKEing and PEEKing into the upper BASIC RAM area and seeing if you get returns matching what you put there.

 

Anyway, I am just flinging poo to see what sticks.  The simplest conclusion here is RAM, and that is ostensibly the simplest thing to test.  If you can get your hands on a good RAM chip or two, you can usually piggy-back a known good chip on suspected bad chip and get it to work properly.

 

I am pretty certain replacement RAM for the 64s is not difficult to find (do not quote me on that.)  IIRC, on most of the full sized boards they are 4164s, for which Jameco has refurbished replacements and there are also several equivalents.

 

Good luck.

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10 minutes ago, bluejay said:

Maybe a bad PLA?

This was one of my thoughts due to certain cartridges not working.  But if BASIC works and some cartridges work, I am leaning toward RAM, first.  I imagine Ray Carlsen's troubleshooting guide would have a solution here.

 

EDIT: Speaking of, check this out: http://personalpages.tds.net/~rcarlsen/cbm/c64/memory.txt

If you get an "out of memory" error or less than the normal number of bytes free (should be 38911 for the C64) when you turn your computer on, that's a RAM problem and usually points to a partial failure in one of the RAM IC chips.

Edited by OLD CS1
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Thanks for everyones input. Ive got a while bunch of carts and disks that I can run through the machine to see whats up. So from what im understanding the pla can affect the ram number? Is it possible all the ram is potentially good, but the pla is causing the number to be lower? 

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2 hours ago, OLD CS1 said:

I wonder if FastLoad bypasses the regular RAM test.  IIRC, the RAM test in the Kernal sets the top of BASIC at the first address that fails to return the same value as what is stored.  FastLoad maybe skip that and make some assumptions.

 

That said, if some cartridges work and others do not, it could still be RAM.  Radar Rat Race may not rely upon RAM in the upper regions which appear to be bad, while Dig Dug might.

 

I believe it is possible, though I am not confident, the problem could be the PLA.  My troubleshooting skills on the 64 are about 20 years rusty -- fortunately, I have not needed to troubleshoot any of mine beyond the standard of a completely dead CIA or PLA.

 

Some disk software may load just fine if the programs do not load up past the 31k mark.  So, something like a simple short BASIC program might work.  Otherwise, something which loads up to, say, 49152, may fail as that is well over the 31k mark.  If BASIC works, you could try POKEing and PEEKing into the upper BASIC RAM area and seeing if you get returns matching what you put there.

 

Anyway, I am just flinging poo to see what sticks.  The simplest conclusion here is RAM, and that is ostensibly the simplest thing to test.  If you can get your hands on a good RAM chip or two, you can usually piggy-back a known good chip on suspected bad chip and get it to work properly.

 

I am pretty certain replacement RAM for the 64s is not difficult to find (do not quote me on that.)  IIRC, on most of the full sized boards they are 4164s, for which Jameco has refurbished replacements and there are also several equivalents.

 

Good luck.

I think i tried loading a small program like circus charlie and that failed to boot. Any disk image thats above 31k would cause it to crash right? So i would need to look for a small program, is there a program you recommend. I have a zoom floppy so i can burn anything to disk pretty much.

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Just now, INTVCruise said:

I think i tried loading a small program like circus charlie and that failed to boot. Any disk image thats above 31k would cause it to crash right? So i would need to look for a small program, is there a program you recommend. I have a zoom floppy so i can burn anything to disk pretty much.

Depends. If a program occupies the bad part of the RAM it won't work; doesn't matter how big a program is. You could try writing your own simple BASIC program, saving, and loading, I suppose. But you're best off testing the RAM.

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35 minutes ago, bluejay said:

Depends. If a program occupies the bad part of the RAM it won't work; doesn't matter how big a program is. You could try writing your own simple BASIC program, saving, and loading, I suppose. But you're best off testing the RAM.

I agree, i think that should be the easiest and cheapest option as well. So if i put a good ram chip on top of another good ram chip while looking for the bad, theres no ill effect to either chip correct so long as they are the same kind of chip in the correct orientation?

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37 minutes ago, INTVCruise said:

I agree, i think that should be the easiest and cheapest option as well. So if i put a good ram chip on top of another good ram chip while looking for the bad, theres no ill effect to either chip correct so long as they are the same kind of chip in the correct orientation?

Depends. If a data pin on the bad ram chip is for some reason conflicting with the same pin on the good ram chip, who knows what shenanigans it might cause.

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21 hours ago, OLD CS1 said:

I wonder if FastLoad bypasses the regular RAM test.  IIRC, the RAM test in the Kernal sets the top of BASIC at the first address that fails to return the same value as what is stored.  FastLoad maybe skip that and make some assumptions.

 

That said, if some cartridges work and others do not, it could still be RAM.  Radar Rat Race may not rely upon RAM in the upper regions which appear to be bad, while Dig Dug might.

 

I believe it is possible, though I am not confident, the problem could be the PLA.  My troubleshooting skills on the 64 are about 20 years rusty -- fortunately, I have not needed to troubleshoot any of mine beyond the standard of a completely dead CIA or PLA.

 

Some disk software may load just fine if the programs do not load up past the 31k mark.  So, something like a simple short BASIC program might work.  Otherwise, something which loads up to, say, 49152, may fail as that is well over the 31k mark.  If BASIC works, you could try POKEing and PEEKing into the upper BASIC RAM area and seeing if you get returns matching what you put there.

 

Anyway, I am just flinging poo to see what sticks.  The simplest conclusion here is RAM, and that is ostensibly the simplest thing to test.  If you can get your hands on a good RAM chip or two, you can usually piggy-back a known good chip on suspected bad chip and get it to work properly.

 

I am pretty certain replacement RAM for the 64s is not difficult to find (do not quote me on that.)  IIRC, on most of the full sized boards they are 4164s, for which Jameco has refurbished replacements and there are also several equivalents.

 

Good luck.

Hey brother quick question, just flipped open the problem C64 again and saw that it looks like the chips are labeled, 4864, so those chips in the link cannot be used as a direct replacement or does it not matter? I have included a picture. Btw, the board is a 250407 Rev A. Thanks for your time. 

problem.jpg

Edited by INTVCruise
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8 minutes ago, INTVCruise said:

Hey brother quick question, just flipped open the problem C64 again and saw that it looks like the chips are labeled, 4864, so those chips in the link cannot be used as a direct replacement or does it not matter? I have included a picture. Btw, the board is a 250407 Rev A. Thanks for your time. 

It appears the 4864 and 4164 should be compatible.  Interesting is that Mitsubishi 4164s are the same refresh as the 4864s, but Fairchild 4164s are 128 cycle.  I have only ever seen them as being interchangeable.  We would need to consult the schematics to see if there are any differences in the RAM circuitry between the various board versions.

 

This is a great resource: http://www.minuszerodegrees.net/memory/4164.htm

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3 hours ago, OLD CS1 said:

It appears the 4864 and 4164 should be compatible.  Interesting is that Mitsubishi 4164s are the same refresh as the 4864s, but Fairchild 4164s are 128 cycle.  I have only ever seen them as being interchangeable.  We would need to consult the schematics to see if there are any differences in the RAM circuitry between the various board versions.

 

This is a great resource: http://www.minuszerodegrees.net/memory/4164.htm

One final question, are the ram chips on an atari 800xl and the commodore 64 swappable? I just realized I had a socketed 800 board that I bought for parts and looking at the ram chips I think they might be a match/compatible?  Again, thank you for your time. 

swap 1.jpg

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48 minutes ago, INTVCruise said:

One final question, are the ram chips on an atari 800xl and the commodore 64 swappable? I just realized I had a socketed 800 board that I bought for parts and looking at the ram chips I think they might be a match/compatible?  Again, thank you for your time. 

Referring to the resource I provided above, the NEC 4164C-15 meets the same specs as the MOSTEK 4164 (MOSTEK = Commodore for our purposes,) so I would give them a try were I a betting man in your position.  As well, with them being in sockets on the 800XL, you will have a much easier time working with them as testing parts.

 

All that said, as you have the 800XL board for parts, do you know the working disposition of these RAM chips?

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4 minutes ago, OLD CS1 said:

Referring to the resource I provided above, the NEC 4164C-15 meets the same specs as the MOSTEK 4164 (MOSTEK = Commodore for our purposes,) so I would give them a try were I a betting man in your position.  As well, with them being in sockets on the 800XL, you will have a much easier time working with them as testing parts.

 

All that said, as you have the 800XL board for parts, do you know the working disposition of these RAM chips?

Thanks for the info, I haven't tested the for parts board yet. I assume it must be either fully working or mostly working. Someone had installed a barrel power jack board and removed the old din connector power jack and also installed an av expansion board on it for audio video. I'm guessing they must have not had the time to finish it or something, but don't really have a way to test it since there is no power switch on it (nor do I know what kind of power source they were planning to use exactly, just that its a barrel jack now, but I suspect they wouldn't have gone through the trouble of modifying it if it was bad). I'll go ahead and pull out one of the ram chips and piggyback to the ram on the C64 board. Hopefully it all goes well, fingers crossed!

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Hey guys, just got ahold of a dead test cartridge so I was able to test out the ram. I let it run for a few cycles, all tests came up okay except the sound test. There are no voices being played on the Sid chip and the filter test also produces no audio. None of the ram spots indicate a bad ram chip, so could this be a problem with the PLA then? Here is a screen shot of the dead test results. 

 

dead 1.jpg

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24 minutes ago, OLD CS1 said:

Taken by itself, that test would compel me to try the PLA and SID.  Given the RAM issues, I would look at the PLA, first.

Thanks, I think I'm going to go ahead and order a spare pla from ebay. I know you can't put Sids form teh c64c into the breadbin model because of the different voltages, is that the case also with PLA's? Does it have to be a specific kind of one for a breadbin vs another model?  

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21 minutes ago, INTVCruise said:

Thanks, I think I'm going to go ahead and order a spare pla from ebay. I know you can't put Sids form teh c64c into the breadbin model because of the different voltages, is that the case also with PLA's? Does it have to be a specific kind of one for a breadbin vs another model?  

PLAs, except for the 64C "short boards," are interchangeable.  The SIDs, as you mentioned, are not: 8580 must be replaced by an 8580, and a 6581 must be replaced by a 6581.  That said, there are modern replacements like the SwinSID, which are relatively inexpensive and interchangeable.

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I would recommend you purchase an ARMSID. They aren’t too expensive, and apparently they sound better than SwinSIDs of the same price range. I haven’t yet tried the one I bought though.

 

This is pretty ghetto, but I’ve heard of people replacing PLAs with cleverly programmed ROM chips. I wouldn’t recommend it, but it might get you limping along until you can find a proper replacement. 

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Unless someone has an abundance of fast EPROMs and a programmer, I think it is just as affordable and a better solution to get one of the multitudes of PLA replacement chips out there. I have lost count how many different solutions there are, and there seems to emerge a new solution every year, usually better or sometimes cheaper than the previous ones.

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